Transmission system of hybrid electric vehicle

ABSTRACT

A transmission system of a hybrid electric vehicle may include an input shaft connected to an engine, a planetary gear set including a first rotation element operated as an output element, a second rotation element connected to the input shaft and operated as an input element, and a third rotation element operated as another output element, a first motor operably connected to the first rotation element, and selectively connected to a transmission housing, a first motor/generator configured to generate electric energy by receiving torque through the first motor shaft, a second motor shaft being a hollow shaft, disposed at a radial exterior of the first motor shaft without rotational interference with the first motor shaft, and operably connected to the third rotation element so as to receive torque of the engine, and a second motor/generator disposed on the second motor shaft and outputting torque through the second motor shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2013-0157667 filed Dec. 17, 2013, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a transmission system of a hybridelectric vehicle. More particularly, the present invention relates to atransmission system of a hybrid electric vehicle which can achieveelectric vehicle (EV) mode, power split mode, and overdrive (OD) mode.

Description of Related Art

Generally, a hybrid vehicle is a vehicle which uses two different powersources efficiently.

Such a hybrid electric vehicle typically uses an engine and amotor/generator. The hybrid electric vehicle uses the motor/generatorhaving relatively better low-speed torque characteristics as a mainpower source at a low-speed and uses an engine having relatively betterhigh-speed torque characteristics as a main power source at ahigh-speed.

Since the hybrid electric vehicle stops operation of the engine usingthe fossil fuel and uses the motor/generator at a low-speed region, fuelconsumption may be improved and exhaust gas may be reduced.

The power transmission system of a hybrid electric vehicle is classifiedinto a single-mode type and a multi-mode type.

A torque delivery apparatus such as a clutch and a brake for shiftcontrol is not necessary, but fuel consumption is high due todeterioration of efficiency at a high-speed region and an additionaltorque multiplication device is required for being applied to a largevehicle according to the single-mode type.

Since the multi-mode type has high efficiency at the high-speed regionand is able to multiply torque autonomously, the multi-mode type can beapplied to a full size vehicle.

Therefore, the multi-mode type instead of the single-mode type isapplied as the power transmission system of a hybrid electric vehicleand is also under continuous investigation.

The power transmission system of the multi-mode type includes aplurality of planetary gear sets, a plurality of motor/generatorsoperated as a motor and/or a generator, a plurality of torque deliveryapparatus controlling rotation elements of the planetary gear sets, anda battery used as a power source of the motor/generators.

The power transmission system of the multi-mode type has differentoperating mechanisms depending on connections of the planetary gearsets, the motor/generators, and the torque delivery apparatus.

In addition, the power transmission system of the multi-mode type hasdifferent features such a durability, power delivery efficiency, andsize depending on the connections of the planetary gear sets, themotor/generators, and the torque delivery apparatus. Therefore, designsfor the connection structure of the power transmission system of ahybrid electric vehicle are also under continuous investigation toachieve robust and compact power transmission system having no powerloss.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing atransmission system of a hybrid electric vehicle having advantages ofachieving EV mode, power split mode, and OD mode according to operationof a brake and a rotation restricting member by combining twomotor/generators, the brake, and the rotation restricting member.

Various aspects of the present invention are directed to providing atransmission system of a hybrid electric vehicle that enhances fueleconomy by not supplying hydraulic pressure to a brake at power splitmode that is used when driving on a city road.

Various aspects of the present invention are additionally directed toproviding a transmission system of a hybrid electric vehicle that mayreduce mechanical power loss and capacity of a motor/generator and mayimprove fuel economy by achieving OD mode using a planetary gear set.

Various aspects of the present invention provides are further directedto providing a transmission system of a hybrid electric vehicle that mayreduce capacity of motor/generator and a length of the transmission byincreasing rotation speed of an input shaft using a planetary gear seton the input shaft and supplying the increased rotation speed to amotor/generator.

According to various aspects of the present invention, a transmissionsystem of a hybrid electric vehicle may include an input shaft connectedto an engine, a planetary gear set including a first rotation elementoperated as an output element, a second rotation element connected tothe input shaft and operated as an input element, and a third rotationelement operated as another output element, a first motor shaft disposedin parallel with and apart from the input shaft, operably connected tothe first rotation element, and selectively connected to a transmissionhousing, a first motor/generator disposed on the first motor shaft andconfigured to generate electric energy by receiving torque through thefirst motor shaft, a second motor shaft being a hollow shaft, disposedat a radial exterior of the first motor shaft without rotationalinterference with the first motor shaft, and operably connected to thethird rotation element so as to receive torque of the engine, and asecond motor/generator disposed on the second motor shaft and outputtingtorque through the second motor shaft.

The transmission system may further include a rotation restrictingmember disposed between the second rotation element of the planetarygear set and the transmission housing and preventing inverse rotation ofthe second rotation element.

The planetary gear set may be a single pinion planetary gear set,wherein the first rotation element is a sun gear, the second rotationelement is a planet carrier, and the third rotation element is a ringgear.

The transmission system may further include a first intermediate outputgear directly connected to the first rotation element, a secondintermediate output gear directly connected to the third rotationelement; a first motor shaft gear engaged with the first intermediateoutput gear and fixedly disposed on the first motor shaft, and a secondmotor shaft gear engaged with the second intermediate output gear andfixedly disposed on the second motor shaft.

The transmission system may further include a speed reduction deviceconfigured to transmit torque of the second motor shaft gear to adifferential apparatus.

The speed reduction device may include an output shaft disposed inparallel with the input shaft, an output shaft input gear fixedlydisposed on one end portion of the output shaft and engaged with thesecond motor shaft gear, and an output shaft output gear fixedlydisposed on another end portion of the output shaft and configured totransmit torque to the differential apparatus.

The transmission system may further include a brake disposed between thefirst motor shaft and the transmission housing.

The rotation restricting member may be a one-way clutch, two-way clutchor a brake.

The transmission system may further include a clutch configured todirect-couple the planetary gear set.

According to various aspects of the present invention, a transmissionsystem of a hybrid electric vehicle may include an input shaft connectedto an engine, a planetary gear set including a first rotation elementoperated as an output element, a second rotation element connected tothe input shaft and operated as an input element, and a third rotationelement operated as another output element, a first motor shaft disposedin parallel with and apart from the input shaft, operably connected tothe first rotation element, and selectively connected to a transmissionhousing, a first motor/generator disposed on the first motor shaft andconfigured to generate electric energy by receiving torque through thefirst motor shaft, a second motor shaft being a hollow shaft, disposedat a radial exterior of the first motor shaft without rotationalinterference with the first motor shaft, and operably connected to thethird rotation element so as to receive torque of the engine, a secondmotor/generator disposed on the second motor shaft and outputting torquethrough the second motor shaft, and a speed reduction device configuredto transmit torque of the second motor shaft gear to a differentialapparatus.

The transmission system may further include a rotation restrictingmember disposed between the second rotation element of the planetarygear set and the transmission housing and preventing inverse rotation ofthe second rotation element.

The planetary gear set may be a single pinion planetary gear set,wherein the first rotation element is a sun gear, the second rotationelement is a planet carrier, and the third rotation element is a ringgear.

The transmission system may further include a first intermediate outputgear directly connected to the first rotation element, a secondintermediate output gear directly connected to the third rotationelement, a first motor shaft gear engaged with the first intermediateoutput gear and fixedly disposed on the first motor shaft, and a secondmotor shaft gear engaged with the second intermediate output gear andfixedly disposed on the second motor shaft.

The speed reduction device may include: an output shaft disposed inparallel with the input shaft, an output shaft input gear fixedlydisposed on one end portion of the output shaft and engaged with thesecond motor shaft gear, and an output shaft output gear fixedlydisposed on another end portion of the output shaft and configured totransmit torque to the differential apparatus.

The transmission system may further include a brake disposed between thefirst motor shaft and the transmission housing.

The transmission system may further include a clutch configured todirect-couple the planetary gear set.

The rotation restricting member may be a one-way clutch, two-way clutchor a brake.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary transmission system of ahybrid electric vehicle according to various embodiments of the presentinvention.

FIG. 2 is an operational chart of the exemplary transmission system ofthe hybrid electric vehicle at each mode according to the presentinvention.

FIG. 3 is a schematic diagram for illustrating power flow in theexemplary transmission system of the hybrid electric vehicle at electricvehicle (EV) mode according to the present invention.

FIG. 4 is a schematic diagram for illustrating power flow in theexemplary transmission system of the hybrid electric vehicle at powersplit mode according to the present invention.

FIG. 5 is a schematic diagram for illustrating power flow in theexemplary transmission system of the hybrid electric vehicle at OD modeaccording to the present invention.

FIG. 6 is a schematic diagram for illustrating power flow in thetransmission system of the hybrid electric vehicle at enginedirect-couple mode according to the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a transmission system of a hybridelectric vehicle according to various embodiments of the presentinvention.

Referring to FIG. 1, a transmission system of a hybrid electric vehicleaccording to various embodiments of the present invention includes aninput shaft IS, a planetary gear set PG, first and secondmotor/generators MG1 and MG2, a brake BK, a one-way clutch OWC that is arotation restricting member, and a speed reduction device CGU.

The input shaft IS is connected to an output side of an engine ENG andreceives torque of the engine ENG.

The planetary gear set PG is disposed on the input shaft IS and is asingle pinion planetary gear set. The planetary gear set PG includes asun gear S, a planet carrier PC rotatably supporting a pinion Pexternally meshed with the sun gear S, and a ring gear R internallymeshed with the pinion P as rotation elements thereof.

The sun gear S is an output element and is directly connected to a firstintermediate output gear MOG1, the planet carrier PC is directlyconnected to the input shaft IS and is connected to a transmissionhousing H through the one-way clutch OWC, and the ring gear R is anoutput element and is directly connected to a second intermediate outputgear MOG2.

The first intermediate output gear MOG1 is disposed at a side of theplanetary gear set PG and the second intermediate output gear MOG2 isdisposed at the other side of the planetary gear set PG.

Each of the first and second motor/generators MG1 and MG2 is operated asa motor or a generator, and are disposed apart from the input shaft IS.

The first motor/generator MG1 is used to generate electric energy andincludes a first stator ST1 fixed to the transmission housing H and afirst rotor R1 rotatable in the first stator ST1.

The first rotor RT1 is directly connected to a first motor shaft MS1disposed in parallel with the input shaft IS. One end of the first motorshaft MS1 is provided with a first motor shaft gear MSG1 fixedlydisposed thereon and engaged with the first intermediate output gearMOG1, and the other end thereof is connected to the transmission housingH by interposing the brake BK therebetween. The brake BK is disposedclose to a rear cover of the transmission housing H.

The second motor/generator MG2 is used to supply driving torque, andincludes a second stator ST2 fixed to the transmission housing H and asecond rotor R2 rotatable in the second stator ST2.

The second rotor RT2 is directly connected to the second motor shaft MS2disposed in parallel with the input shaft IS. The second motor shaft MS2is a hollow shaft and is disposed at a radial exterior of the firstmotor shaft MS1 without rotational interference with the first motorshaft MS1. A second motor shaft gear MSG2 is fixedly disposed on one endof the second motor shaft MS2 and is engaged with the secondintermediate output gear MOG2.

Herein, the second motor/generator MG2 is disposed on a middle portionof the first motor shaft MS1 and the first motor/generator MG2 isdisposed on the other end portion of the first motor shaft MS1.

The speed reduction device CGU includes an output shaft OS disposed inparallel with the input shaft IS and the first and second motor shaftsMS1 and MS2, and an output shaft input gear OIG and an output shaftoutput gear OOG fixedly disposed respectively on both end portions ofthe output shaft OS.

The output shaft input gear OIG is engaged with the second motor shaftgear MSG2 and the output shaft output gear OOG is engaged with a finalreduction gear FG of a differential apparatus DIFF.

The brake BK may be a conventional multi-plate friction element of wettype, and the rotation restricting member may be the one-way clutch OWCbut is not limited thereto. A two-way clutch or a brake may be used asthe rotation restricting member.

FIG. 2 is an operational chart of a transmission system of a hybridelectric vehicle according to various embodiments of the presentinvention at each mode.

Referring to FIG. 2, the transmission system of a hybrid electricvehicle according to various embodiments of the present invention canachieve electric vehicle (EV) mode, power split mode, and overdrive (OD)mode.

The one-way clutch OWC is operated at the EV mode, the brake BK isoperated at the OD mode, and neither of the one-way clutch OWC and thebrake (BK) is operated at the power split mode that is frequently usedwhen the vehicle drives on the city road.

Referring to FIG. 3 to FIG. 5, power flow at each mode in thetransmission system of a hybrid electric vehicle according to variousembodiments of the present invention will hereinafter be described indetail.

FIG. 3 is a schematic diagram for illustrating power flow in atransmission system of a hybrid electric vehicle according to variousembodiments of the present invention at electric vehicle (EV) mode.

Referring to FIG. 3, rotation of the input shaft IS is restricted byoperation of the one-way clutch OWC and the engine ENG is maintained ina stopped state at the EV mode.

At this state, the second motor/generator MG2 is operated and the torqueof the second motor/generator MG2 is transmitted to the final reductiongear FG of the differential apparatus DIFF through the second motorshaft MS2, the second motor shaft gear MSG2, the output shaft input gearOIG, the output shaft OS, and the output shaft output gear OOG.

FIG. 4 is a schematic diagram for illustrating power flow in atransmission system of a hybrid electric vehicle according to variousembodiments of the present invention at power split mode.

Referring to FIG. 4, the power split mode is frequently used when thevehicle drives on the city road, and the brake BK and the one-way clutchOWC are not operated at the power split mode.

The engine ENG and the second motor/generator MG2 are operated and thetorques of the engine ENG and the second motor/generator MG2 aretransmitted to the final reduction gear FG of the differential apparatusDIFF at the power split mode.

That is, the torque of the engine ENG is input to the planet carrier PCof the planetary gear set PG through the input shaft IS, is transmittedto the second intermediate output gear MOG2 as main power through thering gear R, and is transmitted to the first intermediate output gearMOG1 through the sun gear S.

The torque transmitted to the second intermediate output gear MOG2 isadded with the torque of the second motor/generator MG2 at the secondmotor shaft gear MSG2, and the added torque is transmitted to the finalreduction gear FG of the differential apparatus DIFF through the outputshaft input gear OIG, the output shaft OS, and the output shaft outputgear OOG.

The torque transmitted to the first intermediate output gear MOG1 istransmitted to the first rotor RT1 through the first motor shaft gearMSG1 and the first motor shaft MS1 and causes the first motor/generatorMG1 to generate electric energy. The electrical energy generated by thefirst motor/generator MG1 is used to charge the battery or is suppliedto the second motor/generator MG2.

FIG. 5 is a schematic diagram for illustrating power flow in thetransmission system of the hybrid electric vehicle according to thefirst exemplary embodiment of the present invention at OD mode.

Referring to FIG. 5, the brake BK is operated at the OD mode.

The torque of the engine ENG is used as main power and the torque of thesecond motor/generator MG2 is used as an auxiliary power at the OD mode.

That is, if the torque of the engine ENG is input to the planet carrierPC of the planetary gear set PG through the input shaft IS, the sun gearS is operated as a fixed element by operation of the brake BK.Therefore, the torque with increased rotation speed is output throughthe ring gear R. The torque with increased rotation speed is transmittedto the second motor shaft gear MSG2 through the second intermediateoutput gear MOG2 and is added with the torque of the secondmotor/generator MG2 at the second motor shaft gear MSG2. Therefore, theadded torque is transmitted to the final reduction gear FG of thedifferential apparatus DIFF through the output shaft input gear OIG, theoutput shaft OS, and the output shaft output gear OOG.

The transmission system of a hybrid electric vehicle according tovarious embodiments of the present invention can achieve the EV mode,the power split mode, and the OD mode by combining two motor/generatorsMG1 and MG2, the brake BK, and the one-way clutch OWC.

Since the hydraulic pressure is not supplied to the brake BK at thepower split mode that is frequently used when the vehicle runs on thecity road, fuel economy may be improved.

Since the OD mode is achieved by using the planetary gear set PG, fueleconomy may be improved and capacity of the first motor/generator MG1may be reduced.

Since the OD mode is achieved by using the planetary gear set PG, anoutput gear set for achieving the OD mode may be removed and a length ofthe transmission may be shortened.

FIG. 6 is a schematic diagram for illustrating power flow in thetransmission system of the hybrid electric vehicle according to variousembodiments of the present invention at engine direct-couple mode.

Referring to FIG. 6, various embodiments of the present inventionfurther include a clutch CL for direct-coupling the planetary gear setPG.

That is, the clutch CL connects two rotation elements among threerotation elements of the planetary gear set PG and direct-couples theplanetary gear set PG. The clutch CL is disposed between the planetcarrier PC and the ring gear R.

If the clutch CL is operated, the planetary gear set PG becomesdirect-coupling state and the engine direct-couple mode may be achieved.

The transmission system of a hybrid electric vehicle according tovarious embodiments of the present invention can achieve the EV mode,the power split mode, and the OD mode by combining two motor/generators,the brake, and the rotation restricting member.

Since the hydraulic pressure is not supplied to the brake at the powersplit mode that is frequently used when the vehicle runs on the cityroad, fuel economy may be improved.

Since the OD mode is achieved, fuel economy may be improved by reducingmechanical power loss and capacity of the first motor/generator may bereduced.

Since the rotation speed of the input shaft is increased by theplanetary gear set on the input shaft and is then supplied to themotor/generator, the capacity of the motor/generator may be reduced andthe length of the transmission may be shortened.

For convenience in explanation and accurate definition in the appendedclaims, the terms “upper”, “lower”, “inner” and “outer” are used todescribe features of the exemplary embodiments with reference to thepositions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A transmission system of a hybrid electricvehicle comprising: an input shaft connected to an engine; a planetarygear set including a first rotation element operated as an outputelement, a second rotation element connected to the input shaft andoperated as an input element, and a third rotation element operated asanother output element; a first motor shaft disposed in parallel withand apart from the input shaft, operably connected to the first rotationelement, and selectively connected to a transmission housing; a firstmotor/generator disposed on the first motor shaft and configured togenerate electric energy by receiving torque through the first motorshaft; a second motor shaft being a hollow shaft, disposed at a radialexterior of the first motor shaft without rotational interference withthe first motor shaft, and operably connected to the third rotationelement so as to receive torque of the engine; a second motor/generatordisposed on the second motor shaft and outputting torque through thesecond motor shaft; and a rotation restricting member disposed betweenthe second rotation element of the planetary gear set and thetransmission housing and preventing inverse rotation of the secondrotation element.
 2. The transmission system of claim 1, wherein theplanetary gear set is a single pinion planetary gear set, wherein thefirst rotation element is a sun gear, the second rotation element is aplanet carrier, and the third rotation element is a ring gear.
 3. Thetransmission system of claim 1, further comprising: a first intermediateoutput gear directly connected to the first rotation element; a secondintermediate output gear directly connected to the third rotationelement; a first motor shaft gear engaged with the first intermediateoutput gear and fixedly disposed on the first motor shaft; and a secondmotor shaft gear engaged with the second intermediate output gear andfixedly disposed on the second motor shaft.
 4. The transmission systemof claim 3, further comprising a speed reduction device configured totransmit torque of the second motor shaft gear to a differentialapparatus.
 5. The transmission system of claim 4, wherein the speedreduction device comprises: an output shaft disposed in parallel withthe input shaft; an output shaft input gear fixedly disposed on one endportion of the output shaft and engaged with the second motor shaftgear; and an output shaft output gear fixedly disposed on another endportion of the output shaft and configured to transmit torque to thedifferential apparatus.
 6. The transmission system of claim 1, furthercomprising a brake disposed between the first motor shaft and thetransmission housing.
 7. The transmission system of claim 1, wherein therotation restricting member is a one-way clutch, two-way clutch or abrake.
 8. The transmission system of claim 1, further comprising aclutch configured to direct-couple the planetary gear set.
 9. Thetransmission system of claim 6, wherein the brake is disposed close to arear cover of the transmission housing.
 10. A transmission system of ahybrid electric vehicle comprising: an input shaft connected to anengine; a planetary gear set including a first rotation element operatedas an output element, a second rotation element connected to the inputshaft and operated as an input element, and a third rotation elementoperated as another output element; a first motor shaft disposed inparallel with and apart from the input shaft, operably connected to thefirst rotation element, and selectively connected to a transmissionhousing; a first motor/generator disposed on the first motor shaft andconfigured to generate electric energy by receiving torque through thefirst motor shaft; a second motor shaft being a hollow shaft, disposedat a radial exterior of the first motor shaft without rotationalinterference with the first motor shaft, and operably connected to thethird rotation element so as to receive torque of the engine; a secondmotor/generator disposed on the second motor shaft and outputting torquethrough the second motor shaft; a speed reduction device configured totransmit torque of the second motor shaft gear to a differentialapparatus; and a rotation restricting member disposed between the secondrotation element of the planetary gear set and the transmission housingand preventing inverse rotation of the second rotation element.
 11. Thetransmission system of claim 10, wherein the planetary gear set is asingle pinion planetary gear set, wherein the first rotation element isa sun gear, the second rotation element is a planet carrier, and thethird rotation element is a ring gear.
 12. The transmission system ofclaim 10, further comprising: a first intermediate output gear directlyconnected to the first rotation element; a second intermediate outputgear directly connected to the third rotation element; a first motorshaft gear engaged with the first intermediate output gear and fixedlydisposed on the first motor shaft; and a second motor shaft gear engagedwith the second intermediate output gear and fixedly disposed on thesecond motor shaft.
 13. The transmission system of claim 10, wherein thespeed reduction device comprises: an output shaft disposed in parallelwith the input shaft; an output shaft input gear fixedly disposed on oneend portion of the output shaft and engaged with the second motor shaftgear; and an output shaft output gear fixedly disposed on another endportion of the output shaft and configured to transmit torque to thedifferential apparatus.
 14. The transmission system of claim 10, furthercomprising a brake disposed between the first motor shaft and thetransmission housing.
 15. The transmission system of claim 10, furthercomprising a clutch configured to direct-couple the planetary gear set.16. The transmission system of claim 10, wherein the rotationrestricting member is a one-way clutch, two-way clutch or a brake. 17.The transmission system of claim 14, wherein the brake is disposed closeto a rear cover of the transmission housing.